1. Field of the Invention
The present invention relates to fluorine containing polyimide, a novel aromatic diamine which has a perfluoro alkyl radical and can be used as a raw material monomer for the polyimide, and a process for preparing the same. More particularly, the invention relates to a novel thermoplastic polyimide which contains fluorine, has an extremely low dielectric constant and hygroscopic property, and is excellent in processability a preparation process of the thermoplastic polyimide; a novel aromatic diamine compound which has a perfluoroalkyl radical such as trifluoromethyl and is useful as a raw material monomer for the polyimide, for a starting material of polyamide, polyamideimide, bismaleimide and epoxy resin, and as a raw material for organic chemicals; and a process for preparing the aromatic diamine compound.
2. Related Art of the Invention
Polyimide is prepared by reaction of tetracarboxylic dianhydride with diamine. Conventionally known polyimide has an essential characteristic of high heat resistance, is additionally excellent in mechanical strengths, chemical resistance and dimensional stability, and also has flame retardance and electrical insulation property. Consequently, polyimide has been used in electric and electronic fields, particularly in the field where heat resistance is required, and is expected in the future to be used in other application fields and in increased amounts.
Polyimide having excellent characteristics has conventionally been developed. However, conventionally known polyimide has no distinct glass transition temperature though excellent in heat resistance and hence must be processed by such means as sintering in the case of being used as a molded article. In other cases, polyimide is soluble in halogenated hydrocarbon solvents and causes problems on solvent resistance though excellent in processability. Thus, both merits and drawbacks have been found in the properties of polyimide.
Recently, polyimide having improved properties or having a novel performance has been developed in order to extend the utilization fields of polyimide. For example, there has been developed a thermoplastic polyimide of the formula (A): ##STR3##
The polyimide has been disclosed by Proger et al. as a heat resistant adhesive in U.S. Pat. No. 4,065,345. Ohta et al. have provided the polyimide with a new performance of injection ability by controlling the molecular weight of the polymer and capping the reactive end of the polymer chain in Japanese Laid-Open Patent Hei 2-018419.
Development in microelectronics has recently been remarkable in the electric and electronic field. Development research has been extensively conducted in particular on the insulation material for use in a multi-layered circuit substrate. In organic materials applied to the field, polyimide in particular is used as a suitable material for insulation because of excellent heat resistance and dimensional stability and a low dielectric constant as compared with inorganic materials. The dielectric constant of presently marketed polyimide resin is, however, unsatisfactory. For example, polyimide KAPTON (Trade Mark) prepared from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride has a dielectric constant of 3.6 at 1 KHz, the polyimide UPILEX (Trade Mark) prepared from 4,4'-diaminodiphenyl ether and biphenyltetracarboxylic dianhydride has a dielectric constant of 3.5 at 1 MHz, and the polyimide LARC-TP1 (Trade Mark) prepared from 3,3'-diaminobenzophenone and benzophenonetetracarboxylic dianhydride has a dielectric constant of 3.7 at 1 MHz. Polyimide resin has already been used for an insulation material of a flexible printed-circuit substrate. High integration of an electronic circuit has recently been more extended, and accordingly, improvement in electrical characteristics, for example, lowering of the dielectric constant has been strongly desired. Practically, insulation materials having a low dielectric constant of 3.0 or less, preferably about 2.8 have been desired.
Particularly in large-sized computers, high speed transfer of signals by use of 2 multi-layered circuit substrates is inevitable. However, a high dielectric constant of the circuit material leads to a transfer lag of signals and inhibits high speed transfer. Polyimide is used for an interlayer insulation film in multi-layered wiring. Accordingly, attention has been focused to develop polyimide having a low dielectric constant in particular in addition to the above characteristics of conventional polyimide.
Teflon resin has been known as a resin having a low dielectric constant. Investigations for decreasing the dielectric constant has also been carried out on polyimide having heat resistance and other various excellent properties as an engineering polymer. Reduction of a dielectric constant by introduction of fluorine or a fluoro radical into the structure of polyimide has been reported, for example, in A.K. st. Clair et al. Polymeric Materials Science and Engineering, 59, 28.about.32 (1988) and EP 0299865.
That is, introduction of a fluorine atom into the molecular unit of polyimide has been known as a means of reducing the dielectric constant of polyimide. In order to achieve such object, an aromatic diamino compound having a hexafluoroisopropylidene radical has been disclosed as a polyimide monomer used for preparing materials of low dielectric constant (Japanese Laid-open Patent Hei 1-190652).
These aromatic diamine compounds, however, require many steps in preparation and also have problems in industry that the resultant polyimide resin has insufficient melt-flowability in processing.
An aromatic diamino compound which has a biphenyl structure and a trifluoromethyl radical in the molecule is, for example, 4,4'-bis(3-trifluoromethyl-4-aminophenoxy) biphenyl of the formula (B): ##STR4##
The compound, however, has an electron absorbing trifluoromethyl radical in the ortho position to an amino radical and is hence known that the compound is difficult to react with acid anhydride due to an electric factor and is difficult to increase the degree of polymerization.
Since the amino group is located in the para position to the connecting radical, the resulting polyimide has a rigid structure and causes a problem of difficulty in processing.
Other fluorine containing polyimides having low dielectric constant have conventionally been proposed, for example, in Japanese Laid-Open Patent Hei 1-182324, 2-60933, 2-281037 and 4-122729. These polymides are very expensive or difficult to manufacture in industry and it is hence desired to develop a polyimide which is free from these disadvantages and has a low dielectric constant.
Development of plastics having excellent transparency in addition to a low dielectric constant has also been carried out extensively in order to obtain engineering plastics applied to the electric and electronic fields. A plastic which has excellent transparency and is widely used is polycarbonate of the formula (C): ##STR5##
The plastic, however, has low glass transition temperature of about 150.degree. C. and hence heat resistance is unsatisfactory. Another known transparent resin is polyether sulfone of the formula (D): ##STR6##
The resin, however, has a sulfonyl radical which has a high hygroscopic property and thus cannot be used for electric and electronic materials which must be free from moisture.
Further, various kinds of transparent polyimide have also been developed. For example, polyimide having an excellent yellowness index has been disclosed in Japanese Laid-Open patent Hei 1-182324 and has the formula (E): ##STR7##
However, the polyimide also has a problem of hygroscopic property due to the presence of a sulfonyl radical as in the case of the above polyether sulfone.
Problems generally found in polyimide resins have been coloration and higher dielectric constant: than other resins such as Teflon which has a low dielectric constant.
Coloration is a very important problem in the development of optical communication cables and optical materials such as filters and liquid crystals which are used for construction of a highly heat-resistant and reliable communication system. Practically, the yellowness index (hereinafter referred to as YI) is used as a parameter of yellowness. YI is 129 for the polyimide KAPTON (Trade Mark) prepared from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride, 125 for the polyimide UBILEX (Trade Mark) prepared from 4,4'-diaminodiphenyl ether and biphenyltetracarboxylic dianhydride, and 50 for the polyimide LARC-TPI (Trade Mark) prepared from 3,3'-diamino benzophenone and benzophenonetetracarboxylic dianhydride.
All of these YI values are too high. YI of 10 or less is desired for use in the above optical materials. Polyimide resin is desired to have YI of 4.about.48 which is equal to that of polycarbonate for present optical applications.